1. Field of the Invention
The present invention relates to a method of correcting image data picked up from frames photographed on photographic film, especially those photographed by lens-fitted photographic film units or compact cameras, for printing pictures with improved quality on the basis of the corrected image data.
2. Background Arts
As a photographic device for taking pictures on a photographic film, the lens-fitted photo film units have been known these days besides cameras, including single-reflection cameras and compact cameras. The lens-fitted photo film unit, hereinafter called the film unit, is widely used in the world because of its handiness: it contains a roll of unexposed photographic film in a unit body having photographic mechanisms, including a taking lens and a shutter mechanism, incorporated thereinto, to permit enjoying photography quickly on the spot as it is purchased, and the user has only to forward the film unit after photography to an agency of a photofinisher. Since another advantage of the film unit is that it is provided at a low price, the manufacturing cost should be as low as possible, so the structure must be simple, and the components must be inexpensive.
For this reason, the taking lens mounted to the above described film unit is usually composed of a single or a couple of plastic lenses. Were the taking lens is composed of a single or a couple lenses, it is difficult to correct aberrations to improve the image quality only by the performances of the taking lens. For this reason, in the conventional film unit, in order to compensate for curvature of field of the taking lens, one of the factors in deterioration of the image quality, the photographic film is held to curve its surface along its lengthwise direction with the center of curvature located on the object side, i.e. on the side of the taking lens, such that an optical image formed through the taking lens on the curved film surface is in focus in the entire area of an exposure frame that is exposed through an exposure aperture of the film unit.
However, this configuration is ineffective to correct other aberrations, such as chromatic aberration and distortion, and other factors that lower the quality of photographed images. As one of the factors adversely affecting the image quality, it is well-known in the art that image surface illuminance of a taking lens decreases with radial distance from the optical axis of the taking lens. As a result, even where the photographic light entering the taking lens has an uniform brightness, exposure amount is lowered in marginal portions of the exposure frame as compared to a center point of the exposure frame, that is located on the optical axis of the taking lens. Such decrease in illuminance with the radial distance from the center point of the exposure frame cannot be compensated for just by curving the film surface along the lengthwise direction behind the taking lens.
Furthermore, to simplify the structure, the film unit uses a simple shutter mechanism, called kickoff type, that uses a shutter blade swinging in front of a shutter aperture to open and then close the shutter aperture. As shown schematically in FIG. 9, the shutter blade 70 starts swinging from a closing position for closing the shutter aperture 77, when it is kicked by a shutter drive lever. While gradually opening the shutter aperture 71, the shutter blade 70 swings in an opening direction to a maximum opening position as shown by phantom lines. After reaching the maximum opening position, the shutter blade 70 starts swinging in a closing direction according to an urging force of a returning spring 72, while gradually closing the shutter aperture 71, till it returns to the closing position.
In the film unit, the shutter blade is ordinarily located behind a stop aperture at a certain distance from the stop aperture. According to this configuration, rays passing through the taking lens are projected from the stop aperture toward the exposure frame of the filmstrip in the form of a plurality of bundles whose positions are shifted from each other on a plane in which the shutter blade 70 swings, as shown for instance by bundles 73a, 73b, 73c, 73d and 73e in FIG. 9.
Because of the displacement between the bundles 73a to 73e on the swinging plane of the shutter blade 70, in the example shown in FIG. 9, the upper-right bundle 73a first starts entering the shutter aperture 71 and the lower-left bundle 73d last starts entering the shutter aperture 71, as the shutter blade 70 swings in the opening direction. On the contrary, as the shutter blade 70 swings in the closing direction, the lower-left bundle 73d is first blocked from the shutter aperture 71, and the upper-right bundle 73a is last blocked from the shutter aperture 71. As a result, the upper-right portion of the exposure frame is exposed for the longest time, whereas the lower-left portion of the exposure frame is exposed for the shortest time in this example. Consequently, with the kickoff type shutter mechanism, there are differences in exposure time between different positions within the exposure frame.
Indeed it is possible to adopt another type of shutter mechanism that prevents the unevenness of exposure caused by the different exposure time, but such a shutter mechanism is expensive and requires a larger space, so it would increase the cost and size of the film unit. Therefore, this solution is hardly practical.
Meanwhile, digital printing has recently been popular in the world, where pictures are printed on the basis of digital image data picked up through a solid-state imaging device, like CCD. It has also been known in the art to pick up image data from pictures photographed on photographic film, and make photo-prints of the pictures on the basis of the image data. By correcting the image data appropriately before printing, it is possible to obtain good-quality pictures even while the original pictures are photographed by use of a simple and cheap photographic device, such as the film units or compact cameras.
In view of the foregoing, an object of the present invention is to provide a method of correcting image data picked up from pictures photographed on photographic film that is held curved behind the taking lens, so as to compensate for the decrease in illuminance with the radial distance from the center of the exposure frame while taking account of the curvature of the film.
Another object of the present invention is to provide a method of correcting image data picked up from pictures photographed on photographic film by use of a shutter mechanism, so as to compensate for the positional unevenness in exposure time that is caused by the open-close movement of the shutter blade.
According to an aspect of the present invention, a method of correcting image picked up from a frame photographed on a photo filmstrip through a photographic device, comprises the steps of:
calculating an exposure correction amount for a respective pixel of the picture on the basis of correction parameters predetermined with regard to those factors involved in the photographic device which cause unevenness of exposure in the frame, and a relative position of the respective pixel in the frame to an optical axis of a taking lens of the photographic device, assuming that the photo filmstrip is held flat in a focal plane of the taking lens with a center point of the frame located on the optical axis at the photography; and
correcting the image data of each pixel in accordance with the exposure correction amount.
According to another aspect of the present invention, a method of correcting image data picked up from a frame photographed on a photo filmstrip through a taking lens, wherein the photo filmstrip is held concave on the side of the taking lens with a center point of the frame located on an optical axis of the taking lens in a focal plane of the taking lens when the frame is photographed, the method comprising the steps of:
determining a first kind of correction amount for each pixel of the picture, to compensate for decrease in exposure amount that results from decrease in illuminance in a focal plane of the taking lens with radial distance from an optical axis of the taking lens, provided that the photo filmstrip is held flat in the focal plane when the frame is photographed:
determining a second kind of correction amount for each pixel of the picture on the basis of increase in illuminance in those portions of the frame which are shifted from the focal plane toward the taking lens as a result of the curvature of the photo filmstrip behind the taking lens; and
correcting image data of each pixel in accordance with an exposure correction amount obtained from the first and second kinds of correction amounts.
According to a preferred embodiment, the first and second kinds of correction amounts are calculated according to the following equations:
E1=Axc2x7L2+Bxc2x7L4, 
wherein L={square root over ( )}(x2+y2)
E2=Cxc2x7x2+Dxc2x7x4 
wherein E1 and E2 represent the first and second kinds of correction amounts for a pixel picked up from an appropriate point in the frame, L represents a radial distance of the appropriate point from the center point of the frame, xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d respectively represent coordinates values of the appropriate point in orthogonal X-Y coordinates whose plane corresponds to the focal plane, whose X-axis corresponds to the curved direction of the photo filmstrip, and whose Y-axis crosses the X-axis at the center point of the frame, provided that the photo filmstrip is held flat in the focal plane, and A to D represent correction coefficients.
According to a further aspect of the invention, a method of correcting image data picked up from a frame photographed on a photo filmstrip through a taking lens while a shutter mechanism opens a shutter opening, the method comprising the steps of:
calculating an exposure correction amount for a respective pixel of the picture on the basis of correction parameters predetermined in accordance with factors of the shutter mechanism that cause positional unevenness in exposure time within the frame of the photo filmstrip; and
correcting image data in accordance with the exposure correction amount for each pixel.
Where the shutter mechanism comprises a shutter blade that swings about a rotary axis in a perpendicular plane to an optical axis of the taking lens in an opening direction to open the shutter aperture till the shutter blade reaches a maximum opening position past a full opening position where the shutter aperture is fully opened, and thereafter in a closing direction to close the shutter aperture, the exposure correction amount is calculated according to the following equation, assuming that the photo filmstrip is held flat in a focal plane of the taking lens with a center point of the frame located on the optical axis when the frame is photographed:
E3=Kxc2x7log2(Mxc2x7{tanxe2x88x921((Pxe2x88x92x)/(Qxe2x88x92y))}+S)+G 
wherein
E3 represents the exposure correction amount for a pixel picked up from an appropriate point in the frame;
xe2x80x9cxxe2x80x9d and xe2x80x9cyxe2x80x9d represent coordinate values of the appropriate point in orthogonal X-Y coordinates whose plane corresponds to the focal plane of the taking lens and whose axes cross at the optical axis of the taking lens; and
P, Q, S, M, K and G represent the correction parameters.